This proposed research is a continual investigation of auditory signal processing in the mammalian brain. Using bats as a model, an electrophysiological study supplemented with histological confirmation will be conducted to examine the neural mechanism underlying the integration of acoustic spatial information in the pontine nuclei. In auditory signal transmission, the auditory information from the auditory cortex and the inferior colliculus terminate at the pontine nuclei which in turn project their axons as mossy fibers upon the cerebellum. Thus, the pontine nuclei are in a position to integrate messages originating at the auditory system before relaying to the cerebellum, which is believed to be involved in acoustically evoked motor orientation. Preliminary studies in my laboratory have demonstrated that pontine neurons responding to acoustic stimulus are broadly tuned to a wide band of stimulus frequency and often have multipeaked frequency threshold curves. Furthermore, these neurons are not tonotopically organized. These findings strongly suggest that pontine nuclei may encode some stimulus features beyond tonotopy. This proposal hypothesizes that the bat pontine nuclei plan an important role in integrating the processed echo information during the final phase of echolocation. Thus most pontine neurons should b maximally sensitive to a frontal sound source, to a short pulse-echo delay and to a high signal repetition rate. To test this hypothesis, three main electrophysiological experiments will be carried out under both free-field and closed-system stimulation conditions in order to study (1) the auditory spatial sensitivity and binaural properties of the pontine neurons; (2) echoranging properties of the pontine neurons and (3) encoding of signal repetition rate by the pontine neurons. A neural model and the specific procedures to test this model which underlies the formation of two possible types of excitatory-excitatory (EE) neurons in the pontine nuclei are proposed. Furthermore, procedures to explore the neural mechanisms underlying the formation of the best repetition rate encoded by each examined pontine neuron are also described. In auditory orientation, spatial sensitivity and sound localization are of primary importance to any animal. Since the bat relies essentially upon auditory signal processing for survival and its brain structure is fundamentally the same as other mammals including humans, it certainly provides the best model for studying neural mechanism underlying auditory perception. Thus, the results of these proposed studies will not only answer some specific questions regarding echolocation in bats but will also enhance the understanding of sound localization in humans.